Work on the development of C-Plan began in 1995. The idea of an interactive software system to present spatial options for conservation management arose from previous work on irreplaceability in the early 1990s (Pressey 1992, 1999; Pressey et al. 1993, 1994b) (Chapter 2). At that time, this research on irreplaceability added a new dimension to the problem of selecting indicative sets of sites to achieve quantitative targets for features such as vegetation types or species (the set covering problem, Camm et al. 1996). The basic ingredients for the set covering problem are: (a) planning units, the sites to be assessed and compared as potential conservation areas, (b) maps of biodiversity features, (c) a target for each feature (e.g. number of hectares of each vegetation type and number of locality records for each species), and (d) a data matrix listing the extent or occurrence of each feature in each planning unit (see Chapter 3). Using these same ingredients, irreplaceability was conceived and implemented as a solution to an important limitation of reserve selection software at the time. The limitation was that any selected set of sites required to achieve targets is usually only one of the many possible sets, all of which differ to some extent in their composition and configuration of individual sites.